In the art of two-rotor continuous mixers for plastic materials, it is known to make a mixing chamber barrel (also called a "mixing chamber housing") in two parts: an upper half and a lower half with a horizontal split line (parting line) between the upper and lower halves. Often the upper half of the mixing barrel is hinged to the lower half near their horizontal split line so that the upper half can be swung upwardly away from the lower half in clam-shell manner for opening up the mixing chamber.
After the mixing chamber has been opened, it is difficult to perform a thorough and complete cleaning of the interior surface of the mixing chamber and/or to carry out thorough and complete maintenance of this interior surface, because in the prior art two one-piece rotors extend side-by-side for the entire length of the mixing chamber and thus they block access to the interior surface of the lower half of the mixing chamber. Intentionally, there is very small clearance between the interior surface of the mixing chamber and the tips of the helical screw flights and tips of the mixing wings of the rotors, because the dynamic interactions between the wall surface of the mixing chamber and the screw flights and mixing wings of the rotors operating on plastic materials within this small clearance generate and create the desired forward pumping action of the screw flight and the desired mixing of the ingredients of the plastic materials.
It will be understood that such small clearance causes the two side-by-side rotors extending for the length of the lower chamber half substantially to block access to the interior surface of the lower chamber half and to block access to the undersides of both rotors, unless they are laboriously turned in an attempt to rotate their under surfaces to the top providing some exposure. To fully clean these areas, the prior art one-piece rotors would need to be lifted out of the mixer housing by disassembling both bearing assemblies, a chore that is neither welcomed nor recommended. In summary, access to the inner surface of the bottom half of the mixing chamber is substantially entirely blocked in the prior art by two one-piece rotors, and access to the rotors themselves is considerably impeded.
In the plastics materials industry there are now numerous additives available for plastics, and new types of polymers and copolymers are becoming commercially available, such that many formulations and mixtures of plastic materials have not yet been tried out by potential users. Further, there are increasing requirements for engineered plastic components and for medical plastic components providing high qualities of mechanical performance and having tight specifications on chemical compositions.
Consequently, users will find benefits in customizing the mixing of their plastic materials. For custom mixing of plastics, users may wish to operate mixers with relatively short production runs for making various plastics formulations for various marketing opportunities. Attempting to obtain relatively short production runs of customized plastics formulations from third-party commercial processing organizations involves considerable expense, because a commercial processing organization inherently prefers to make relatively long production runs for maximizing productivity, since considerable labor is involved in clean-out of the mixing equipment prior to mixing a customized plastics formulation. Careful clean-out is needed to avoid contamination of a formulation by residues of previously processed plastic materials. Moreover, if the clean-out is not complete, some unexpected and undesirable contamination may occur.
In order for a user efficiently and successfully to operate sequences of relatively short runs of various customized plastics formulations, the user will want to make thorough and complete clean-outs of the mixing chamber and of the two rotors subsequent to each run and prior to commencing the next run, thereby being sure to avoid contamination of subsequent formulations by residues of prior formulations.
In addition, for a user to mix a predetermined plastics formulation this month with specific desired properties and then reliably to mix the same formulation with precisely the same specific desired properties several months from now, the user will want to be able to carry out maintenance on the mixer and its rotors for assuring that the equipment is in tip-top shape with rotor surfaces and the interior surface of the mixing chamber having specified clearances and specified surface finish characteristics. For reasons described above, prior art two-rotor continuous mixers having one-piece rotors present impediments to maintenance of the surfaces of the two rotors and of the interior surface of the lower half of the mixing chamber.